The invention relates to a gear pump, especially for a power steering system, having a housing, a first cover and a second cover.
As a rule, such pumps consist of many individual parts such as gaskets, gear wheels, bearing shafts, bearing rests, centering pins, etc., so that their assembly is very demanding. The installation of the pump, for example, into a motor-pump aggregate of a power steering system of a motor vehicle is usually done in an awkward manner using screws or similar fasteners.
Document DE 203 02 535 U1, and corresponding US Publication No. 2006/0051230, describes a hydraulic pump of the generic type that can be used as a prefabricated assembly in a motor-pump aggregate of a power steering system. The end faces of the housing are each closed by a cover, the two covers being held together by at least one holding part. The hydraulic pump is accommodated in a fluid-filled chamber that is pressurized either by the pump itself or by another means. Therefore, when the individual parts of the pump are designed, it can be assumed that the pressure contributes to holding together the individual parts among each other as well as to affixing the pump as an assembly. In this situation, only a small contact pressure of the individual parts is needed to ensure the start-up of the pump. This contact pressure is necessary in order to seal the intake and pressure areas of the pump. Due to the external pressurization of the housing and of the pump cover, there is only little force differential or no force differential at all between the interior of the pump and the space surrounding the pump, so that the wall thicknesses of the above-mentioned components of the pump can be minimized. The configuration of the pump as a prefabricated assembly has the advantage that each pump can be tested before being delivered or before being installed and that the installation as an assembly is much simpler than the installation of numerous separate individual parts. Moreover, when a pump that has been configured in this manner is assembled, it is not necessary to join the two covers individually to the housing. The holding part ensures that the two covers are pressed against the housing without the need for further affixing means. In the publication cited, either screws or a spring element are used as the holding part, the spring element offering the advantage that the height of the covers can be reduced since there is no need for a thread and no screw head has to be accommodated.